Cold Frames, Greenhouses, and Beyond: Four-Season Gardening

Help extend your gardening efforts into all four seasons by exploring cold frames, greenhouses and hoop houses to enjoy fresh veggies all year long.

article image Howden
Digging in during the winter months.

Learning to garden in four seasons – rather than one (summer) and fragments of two (spring and fall) – can be a bit like learning to think in four or five dimensions. But the rewards, say Colorado gardeners Rick and Shirley Visser, are mind-boggling abundance: fresh salads in December, carrots in February, spinach in March, and a whole new appreciation of what’s possible.

“Some people want to plant a garden and have it over with. But I’ve had more produce in fall than I’ve ever had in summer,” Visser says.

For Adam Montri, who, with his wife, Dru, and young daughter, Lydia, runs Ten Hens Farm in Bath, Michigan, the difference a high-tunnel hoop house made on their farm was the ability to reap an income year-round rather than just for the summer season.

In his job educating about hoop houses, a joint project of Michigan Food and Farming Systems and Michigan State University, Montri also noticed a less tangible, but infinitely richer, change.

“You always get a community growing around the farmers’ markets,” he says. “This makes that community happen year-round.”

For many farmers in the upper two-thirds of North America, planting under cover – whether that cover is as minimal as compost-heated planting beds with floating row covers or as intricate as a 30-by-90-foot greenhouse erected over cherry trees – can alter an operation’s destiny and the farmer’s relationship with the land.

The Vissers became four-season gardeners when they moved from a higher elevation down to Longmont, a town outside Boulder, Colorado. Another daughter was selling her small, turn-of-the-century home with an extra-deep lot, and Rick had long craved the space, soil and less harsh climate. When remodelers came to make changes to the house and make his garage an art studio, he had them rework the yard as well – incorporating six raised beds, a mini-orchard and composting area, a cold frame and two sheds.

That cold frame is now Visser’s pride and joy, much more so than a small kit greenhouse he attached to the warm west side of one of the garden sheds. Between the two, he’s able to grow sturdy tomato seedlings that germinate in the home’s basement and produce a crop of low-growing vegetables that feed the couple through the entire winter. The cold frame’s low height – only 6 to 10 inches above the ground, with interior soil levels below that – is sufficient to keep his crops growing. And the operation is low maintenance: He props each pane of glass up with a notched 2-by-2 in the mornings and closes them at night.

Sure, you might say, that’s possible in Colorado, with 5,000-feet-plus elevations and 300-plus days of sunlight per year to warm crops in covered beds. But what about cloudy Michigan? The hills of Pennsylvania? Wind-scoured Oklahoma? Yes, oh, yes, and, absolutely, yes.

Raised Garden Boxes

Coleman, who can truthfully be called one of this revolution’s leaders, farms year-round in Maine’s USDA Hardiness Zone 5, not exactly known for sunny winter days and with lows of 10 to 20 degrees below zero. Pennsylvania State University is a hotbed of hoop-house research. And in Oklahoma – a relatively balmy Zone 7 – interest in season extension has snowballed, says an agriculture foundation expert.

By selecting cold-hardy crops and varieties within those crops, by understanding the effect of well-timed and successive plantings to correspond with crucial day-length thresholds, and by using one or two levels of climate protection, the farm is able to produce market crops of superb quality and freshness 47 weeks of the year. (After assessing market demand, profitability and the need for some fallow time for themselves, they decided to set aside five weeks for a well-earned vacation.)

Four Seasons Farm, which Coleman operates with his wife, Barbara Damrosch, represents the opposite end of the complexity and size spectrum from the Vissers’ home garden. The farm sells a variety of field-grown vegetables throughout the warmer months, but is able to get an extra-early start with heat-loving crops, such as tomatoes, using a combination of a seed-starting greenhouse and a minimally heated “cool house” that becomes warmer as summer nears.

Key to the farm’s other-than-summer sales are four other structures: moveable, unheated greenhouses in which cold-hardy crops are grown in the soil and under a secondary, spunbond cover about 12 inches above the soil. Radishes, lettuce, mache, and a “stir-fry pack” of pak choi, carrot, radish, leek and “Hakurei” turnip with greens not only grow well in these cool conditions, he writes, they have excellent eating quality. If you visit them at the night’s lowest temperature, he says, the plants look as though they’ll die from frost by morning – but once the sun is truly up, they’re alive, vibrant and growing.

“The winter harvest would appear to have endless potential for all of us involved in exploring it. We are just beginning to tease out the possibilities,” Coleman writes.

There are winter-conquering strategies between the two extremes of cold frames and high-tunnel hoop houses. Jeff Ashton’s book came about because he was an inveterate tinkerer.

“It’s in a guy’s nature to try to reinvent the wheel. And I was obsessed with gardening, so I read everything I could get my hands on. I read the current stuff, and then I went backwards. In the past, gardeners have had to come up with strategies to grow things fresh, out of season, if they wanted to have them.”

At that time, Ashton’s roots were planted in Asheville, North Carolina, where cold, humid winds can tumble down the mountains, sometimes bringing snow. He gradually decided to grow all of his seedlings in his cold frame. “They come up slower, but they come up stronger, and they’re already hardened off. Less is more – and less work is definitely more.” His less-work techniques for smaller-scale gardens include lampshade-frame, paper cloches, low hoops and row covers.

Ashton’s 2001 manual, now out of print, harbors construction-guy plans and detailed hardware recommendations for cold frames, low tunnels, “stoop houses,” and larger, but still small-scale, economical structures. He reminds gardeners to use such techniques to extend the harvest season, not to try to grow vegetables that are radically out of season. Tomatoes in April? No. But overwintered carrots and salsify roots? Yes. Kale, parsnips, brussels sprouts planted in summer, and harvested throughout winter? Absolutely, deliciously yes.

High hoops

If after thoroughly researching high-tunnel hoop houses – or other walk-in, season-extending structures – you decide to take the plunge, experts advise proceeding at all deliberate speed. Depending on size, it’ll be a significant investment, though nowhere near the cost of a greenhouse. And there is a downside: late August to mid-October gets busier, because along with the usual summer harvests, you’re seeding winter crops.

Site preparation – about an 18-month process – is absolutely key, Montri says. “It’s a lot easier to get the weeds out before you build the house.” He recommends sowing your chosen site with a crop of winter rye in fall. Let it overwinter, then frost-seed it with clover to grow in the shade of the rye. Till in the clover in summer, then amend the soil for winter and early spring harvest.

While taking the time to research and prepare to build a hoop house may be frustrating, it has the benefit of forcing you to learn from colleagues in your area. Farmers don’t have to go it alone. Grants, workshops and other learning opportunities centered around season extension are springing up just about everywhere winter exists.

Steve Upson, a season-extension expert at The Samuel Roberts Noble Foundation headquartered in Ardmore, Oklahoma, has been working with growers and high-tunnel hoop houses since 1995, but “the last two years with the recession, the demand has just been insane” for better, more productive food-growing techniques.

Gardeners need to assess their goals and, if they’re selling to market, what crops are likely to be most profitable for them. That might be cut flowers rather than vegetables – or tomatoes and specialty peppers rather than easier-to-grow, lower-priced green beans. Or cherry trees. Upson recommends checking out High Tunnels for photos, research and case studies.

“If you’re really into winter harvest, two small houses are better than one big one. That way you can let one get really warm to get a head start on warm-season crops, and keep the cool-season crops in the other one,” he says.

Homemade Cold Frame

This summer, Coleman held classes at Seed Savers Exchange in Decorah, Iowa, where two high-tunnel hoop houses were built. Montri also spent time in Iowa, doing a high-tunnel workshop for Practical Farmers of Iowa, where season extension ranks first in the organization’s horticulture research priorities.

“In the last three years, we’ve probably put up 30 to 35 high tunnels. We just got a call from a county that got a USDA grant to put up 10,” he says. Michigan State staff will likely do a training build in that county, where one structure will be put together with labor drawn from area farms. “It’ll take longer to build, but everyone will learn how to do it.”

That process, he adds, sows seeds for a different kind of year-round harvest.

“Working with people who don’t have a lot of construction experience, the excitement and confidence that they get – it’s like an old-fashioned barn-raising. You see this change happen in people, and my perception is that it goes beyond building things.”

Susan Clotfelter gardens in northern Colorado, is a food and feature writer for The Denver Post and a Master Gardener volunteer for the Colorado State University Extension.

Try These for Cold-season Harvests

  • Beets (greens): Bull’s Blood
  • Carrots: Napoli, Sugarsnax
  • Kale: Red Russian, Winterbor, Toscano, Dinosaur
  • Asian greens: Mei Quin and Joy chois, Mizuna, Mustard greens
  • Lettuces: Mache, Marvel of Four Seasons, Aruba, Ermosa, Winter Density, Bianca Riccia
  • Radishes: Easter Egg, Avignon
  • Swiss chard
  • Spinach
  • Turnips: Hakurei, for roots and greens

The Easy Squash Hothouse

A favorite season-stretching trick of Jeff Ashton, author of The 12-Month Gardener, is planting winter squash in his compost piles. “A three-week head start on winter squash will give you significantly more squash,” he says.

Ashton composts in frames built out of used wooden pallets, so his squash beds are enclosed, but also several feet deep. Two to three weeks before the last frost, he picks one that’s fairly warm. (If your compost isn’t warm, inoculate and water regularly to keep it active.) Firm up good, rich garden soil in a 2-gallon pot, and dig a hole in the compost that’s large enough to fit that much soil.

Dump the soil into the compost pile, without the pot. Repeat twice, to have three soil “pots” in a 4-foot-square compost pile, and plant three squash seeds within each “pot.” Don’t worry if the compost is fairly fresh because you want the heat from its decomposition to warm the soil. Cover the compost pile with a floating row cover weighed down with straw. Heat from below gets the seedlings started; as their vines and roots begin to take off, they’ll be growing in a pile of squash food. When it looks like the squash needs water? That’s when the compost needs water as well.

“You get a bumper crop of squash and a free compost water gauge,” Ashton says. “And you get to save the more important garden real estate for other things.”

Season-Extension Tools and Terms

Cold frame – a box, usually sunk into the ground a foot or so, with an angled, transparent lid that can be opened, closed or propped partly open to vent heat. The bottom can be open to soil or filled with gravel; in-ground plants can be seeded or seedlings hardened off in pots. Also known historically as Dutch lights.

Cloche – an old French market-gardener’s term for a bell-shaped glass jar placed over young plants in spring. Wall-o-waters and hot caps are the current incarnations.

Floating row cover – water and air permeable, these synthetic fabrics help hold in the heat when placed close to the ground. The transparent plastic version, placed right over the soil and held down with stakes or weights, can create a mini-microclimate for seed germination. Outdoors, the covers can help keep out insect pests.

Quick hoops, low hoops, low tunnels – slightly higher coverings, stretched across frames over outdoor crops that still provide a single layer of climate extension. They can be added only to protect from spring and autumn frosts or employed year-round if vented.

Hoop house, high tunnel, cold house, cool house – unheated or minimally heated (to just above freezing) structures, tall enough to stand in, where crops are grown in-ground.